The performance of certain types of MOS transistors may be improved by the presence of strain in the channel region.
In particular, in a P-channel MOS transistor having a SiGe silicon-germanium channel region, directed along crystal direction <100>, the mobility of holes is increased by compressive strain oriented along the drain-source direction, that is, the direction of the transistor length. However, compressive strain in the direction of the transistor width decreases the mobility of holes. It is desirable to increase the mobility to increase the transistor speed.
Known methods to form strained transistors raise various problems, particularly in the case of transistors having very small dimensions, formed inside and on top of active areas having a length shorter than 400 nm.